Field of the Invention
The present invention relates to an image forming optical system and an electronic image pickup apparatus using the same.
Description of the Related Art
In recent years, due to an increase in the number of pixels of an image pickup element and the progress in a digital image processing technology, silver-salt 35 mm film cameras have been replaced by digital cameras. Moreover, since the number of pixels of a small-sized liquid-crystal panel which is used as a finder has increased, even interchangeable lens cameras are undergoing transition from so-called single lens reflex cameras to cameras of a new concept (mirrorless single lens cameras) in which, a quick return mirror has been eliminated.
By elimination of the quick return mirror, designing of an image pickup optical system having a small size and an improved performance, with a short back focus to certain extent, has become possible. A constraint that a size of a finder image which is achieved depends on a size of an image formed by a lens, or in other words, an image circle diameter, is not there anymore. Therefore, a large finder image is achieved even if an image pickup format size is small, and moreover, designing of an image pickup optical system having a small size and an improved performance has become possible.
Incidentally, as a camera with a short back focus and small image forming format, fixed lens small cameras commonly called as compact cameras are in widespread use. A zoom lens appropriate for compact cameras has been described in Japanese Patent Application Laid-open Publication Nos. 2002-48975, 2002-365545, and 2003-140043. The zoom lens in each of Japanese Patent Application Laid-open Publication Nos. 2002-48975, 2002-365545, and 2003-140043 includes in order from an object side, a lens unit having a negative refractive power, a lens unit having a positive refractive power, and a lens unit having a positive refractive power.
Among zoom lenses of various types, an arrangement of a zoom lens of the most basic type is a two-unit arrangement which includes in order from an object side, a lens unit having a negative refractive power and a lens unit having a positive refractive power. The zoom lens in each of the Japanese Patent Application Laid-open Publication Nos. 2002-48975, 2002-365545, and 2003-140043 is a zoom lens in which, a third lens unit having a positive refractive power is added on an image side of the zoom lens having the two-unit arrangement. If the number of lens units is increased, a total length of the optical system is prone to become long at a wide angle end. However, by increasing the number of lens units, a high zooming ratio is achieved while suppressing the increase in the total length of the optical system.
Even if the number of lens units is increased to more than three, an effect of shortening the total length of the optical system is small. A zoom lens in which, the number of lens units is let to be four is described in Japanese Patent Application Laid-open Publication Nos. 2003-131130, 2008-233611, and 2012-22106. The zoom lens in each of Japanese Patent Application Laid-open Publication Nos. 2003-131130, 2008-233611, and 2012-22106 includes in order from an object side, a lens unit having a negative refractive power, a lens unit having a positive refractive power, a lens unit having a negative refractive power, and a lens unit having a positive refractive power. The zoom lens in each of Japanese Patent Application Laid-open Publication Nos. 2003-131130, 2008-233611, and 2012-22106 has been proposed as a zoom lens for a compact camera. However, eventually, the zoom lens of the three-unit arrangement has been popular.
As aforementioned, by eliminating the quick return mirror, in the optical system according to the camera of a new concept, the back focus could be made short. However, as long as the optical system of the camera of a new concept is an interchangeable lens optical system, the fact remains that it is important that a total thickness of lenses, rather than the total length of the optical system, is small. Moreover, there is a restriction also on an amount of the optical system protruding from a flange surface (lens installation surface). Given this situation, it is necessary to make the back focus of the optical system of the camera of a new concept longer than the back focus of the optical system for a compact camera.
As aforementioned, among zoom lenses of a compact camera, there are zoom lenses in which, an arrangement of refractive power of lens units is in order of a negative refractive power, a positive refractive power, and a positive refractive power, from an object side, and zoom lenses in which, an arrangement of refractive power of lens units is in order of a negative lens unit, a positive lens unit, a negative lens unit, and a positive lens unit. In the zoom lens with the refractive power arrangement in order of a negative refractive power, a positive refractive power, and a positive refractive power from an object side, if the back focus is made long, small-sizing and slimming of the optical system becomes difficult. Whereas, in the zoom lens with the refractive power arrangement in order of a negative refractive power, a positive refractive power, a negative refractive power, and a positive refractive power, even if the back focus is made long, small-sizing and slimming of the optical system is possible.
Zoom lenses with a refractive power arrangement in order of a negative refractive power, a positive refractive power, a negative refractive power, and a positive refractive power, in which, the back focus is made long to some extent, are disclosed in Japanese Patent Application Laid-open Publication Nos. 2012-58406 and 2012-133230.